Organic solar cells and fuel cells that are currently being used are comprised of expensive polymers and are inefficient because much of the energy is lost in the production process. A solar cell (also referred to herein as a “photovoltaic cell”) is any device that directly converts the energy light (i.e., light energy or photons) into electrical energy through the process of photovoltaics, which is also referred to as the “photovoltaic effect.” A solar cell is a specialized semiconductor diode that converts visible light into direct current (DC) electricity. Some photovoltaic cells convert infrared (IR) or ultraviolet (UV) radiation into DC. A photovoltaic module is a packaged, connected assembly of solar cells. A solar panel is a set of solar photovoltaic modules electrically connected and mounted on a supporting structure. The majority of solar modules use wafer-based crystalline silicon cells or thin-film cells based on cadmium telluride or silicon. Most solar modules are rigid, but semi-flexible ones are available, based on thin-film cells.
A common characteristic of both the small molecules and polymers used in photovoltaics is that they all have large conjugated systems. When these materials absorb a photon, an excited state is created and confined to a molecule or a region of a polymer chain. The excited state can be regarded as an electron-hole pair bound together by electrostatic interactions, i.e. excitons. In photovoltaic cells, excitons are broken up into free electron-hole pairs by effective fields. The effective fields are set up by creating a heterojunction between two dissimilar materials. Effective fields break up excitons by causing the electron to fall from the conduction band of the absorber to the conduction band of the acceptor molecule. For the heterojunction to function, the acceptor material must have a conduction band edge that is lower than that of the absorber material.
Currently, most commercial solar cells are made from a refined, highly purified silicon crystal, similar to the material used in the manufacture of integrated circuits and computer chips (wafer silicon). The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies such as polymer solar cells.
A polymer solar cell is a type of flexible solar cell made with polymer chains formed from large molecules with repeating structural units that produce electricity from sunlight by the photovoltaic effect. Polymer solar cells include organic solar cells (also called “plastic solar cells”). They are one type of thin film solar cell; others include the currently more stable amorphous silicon solar cell. Polymer solar cell technology is relatively new and is currently being very actively researched.
Compared to silicon-based devices, polymer solar cells are lightweight, potentially disposable and inexpensive to fabricate, flexible, customizable and they have lower potential for negative environmental impact. The major disadvantage of polymer solar cells is that they offer about one-third of the efficiency of hard materials. They are also relatively unstable toward photochemical degradation. For these reasons, despite continuing advances in semiconducting polymers, the vast majority of solar cells rely on inorganic materials.
Organic polymer solar cells (“OPSCs”) differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction (i.e., the boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor) to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices.
A typical bulk heterojunction solar cell consists of an anode, followed by a hole-transporting layer, then the active layer, followed by the cathode. Typically, the anode and cathode are composed of indium tin oxide and aluminum, respectively. The hole-transporting layer is composed of two ionomers; the negatively-charged sodium polystyrene sulfonate (“PSS”), and the positively-charged poly(3,4-ethylenedioxythiophene) (“PEDOT”).
An organic photovoltaic (“OPV”) cell is composed of a film of organic photovoltaic active layer, sandwiched between a transparent electrode and a metal electrode. Typically, the active layer of a polymer solar cell (“PSC”) device is composed of a blend film of conjugated polymer (as electron donor) and a small molecular acceptor. The conjugated polymer donor and the fullerene derivative acceptor are the key photovoltaic materials for high performance PSCs. Identifying the ideal properties and selecting photovoltaic materials with these ideal properties are important factors in photovoltaic materials design.
In bulk heterojunction polymer solar cells, light generates excitons. Subsequent charge separation in the interface between an electron donor and acceptor blend within the device's active layer. These charges then transport to the device's electrodes where the charges flow outside the cell, perform work and then re-enter the device on the opposite side. The cell's efficiency is limited by several factors, especially non-geminate recombination. Hole mobility leads to faster conduction across the active layer.
Attempts have been made using various approaches for integrating graphene into solar cells. In some cases, a multi-layer graphene was used as an electrode and in another case a thin layer of UV-Ozone-treated gold was placed onto a multi-layer graphene to make an anode. Another attempt was made using amino acid glycine as an environmentally friendly reducing reagent for the synthesis of gold nano-particles-graphene oxide (“AuNP-GO”) nanocomposites, which were then used in organic photovoltaic cells (“OPVs”). However, Raman spectroscopy showed that the glycine only reduced the Au but not the graphene oxide to graphene. Hence, the Au-graphene oxide was incorporated into the OPVs but not the graphene. In another attempt, GO and AuNP-GO solutions were “spincoated” onto indium-tin-oxide (“ITO”) glass substrates, which were then used as a layer in a solar cell.
The performance of any type of solar battery is closely dependent on the conductivity of the electrode material and the adhesion of the electrode to the solar cell film. It has recently been proposed to use graphene and graphene oxide for the cathode and anode respectively since the two materials have appropriate work functions for the hole and the electron, respectively, and are resistant to oxidation. Adhesion of the graphene materials to the polymer films, though, still poses a problem. The polymer surface of the OPSC is hydrophobic and, hence, not amenable to any water-lift off technique for processing or plating. Spin coating is problematic since hydrophobic organic solvents sometimes used to disperse the graphene also dissolve the underlying polymer film and disturb the surface properties. Chemical vapor deposition of graphene at 1000° C. also degrades the underlying active polymer layer. Therefore, there is a need for a method for easily and economically incorporate graphene in a polymer layer. There is also a need for a polymer solar cell that can generate electricity more efficiently than the polymer solar cells now being used.